In actual practice they didn't want to know or do anything about the legality or illegality of the transactions they were facilitating because a large portion of their draw is the the hands off nature. Its likely that a good chunk of the ads were in fact for over 18 individuals but neither we nor backpage probably know how much was illegal.

That is neither a sound logical argument, nor is it evidence towards your point.

And to answer your question, Yes. Do you have much familiarity with the various systems in which child sex trafficking happens in the US? If not, that ignorance may be a huge factor towards the dissonance you're displaying.